Military

Further Reading

Hydrofoil Limitations

The tonnage of hydrofoil type craft is limited currently to about 400 tons, because the weight increase of a hydrofoil craft is much faster than the lifting force from foils when the dimensions of the vessel are increased. The tonnage increases cubically by its dimensions while the lifting force from the foils increases squarely by its dimensions. The power transmission system on retractable foil type hydrofoil craft is very complicated and often causes maintenance and operational problems resulting from many delicate moving elements enclosed in the movable spidery struts. It is very inconvenient for a fixed foil type of hydrofoil craft to rest at a limited depth of harbor because the extension of foil planes may hit the bed of the harbor: especially for a large tonnage vessel with long and deep fixed foils.

The various hydrofoils have been found to be deficient in several respects. For example, the operation of hydrofoil vessels was easily impaired by obstructions, such as floating logs and pilings, grasses, weeds, shallow water, rocks, sandbars, mudflats, and shores, and it was difficult to adapt the vessels for landing on and launching from beaches and in general for amphibious use. At higher speeds associated with hydrofoil boats, collisions between the foils and submerged objects are of great concern. Prior art hydrofoils include hinged struts and other methods of releasably rotating the foil support structure, should the foil strike a submerged object. Unfortunately, designs do not provide for safe transition to hull borne operation and the danger of cartwheeling or otherwise capsizing is high; particularly in a smaller craft at high speed. Should any main foil of the subject boat strike a submerged object; shearing fasteners fail, allowing the foil to swivel and streamline, converting to a ski; thus permitting a safe, stable deceleration of the craft.

Another problem resulted from the operation of the vessel in heavy seas or other conditions involving substantial wave action. The rigidly mounted hydrofoils previously employed produce rough rides or instability, and it was difficult to mount an appropriate suspension between the main portion of the vessel and the foils in an economical manner.

Still further difficulties in the operation of conventional hydrofoils resulted from excessive frictional drag due to the movement of their wetted surface area through the water, with the corresponding deleterious effect on the speed of the vessel. Attempts to reduce the wetted surface area have met with the problem that, at lower speeds, the reduced lifting effect did not adequately support the vessel. As a result the drag of the foils plus the drag of the hull was substantially more than the drag of the hull alone would have been.

Conventional hydrofoil craft have a number of problems which make them difficult or impractical to operate at high speeds. A first problem is cavitation, which is a phenomenon in which vapor bubbles form along the upper surface of a foil due to a low fluid pressure on this surface. Cavitation invariably occurs in conventional hydrofoil craft above a certain operating speed (typically around 50 knots). When the vapor bubbles caused by cavitation collapse in the water, they produce strong shock waves. If the collapse occurs in the vicinity of the foil, the shock waves not only produce unpleasant noise and vibrations, but can also physically damage the foil of the craft by pitting.

In order to prevent damage by cavitation, foils referred to as supercavitating foils have been developed. With a supercavitating foil, a large vapor-filled cavity, referred to as a separation bubble, is formed over substantially the entire upper surface of the foil. Vapor bubbles in the cavity are carried beyond the trailing edge of the foil and collapse in the water aft of the foil, so that shock waves produced by the collapse of the bubbles have much less effect on the foil than in a normal cavitating foil.

While a supercavitating foil prevents the collapse of air bubbles in the vicinity of the foil which could damage the foil, in order to prevent the separation bubble from collapsing, it is necessary to maintain the foil at an extremely high angle of incidence. This high angle of incidence results in a great deal of drag, so that the lift/drag ratio of a conventional supercavitating foil is so low as to make such a foil impractical. For this reason, supercavitating foils are not used in practice, and hydrofoil craft must rely on conventional cavitating foils, which as described above are unsatisfactory.

Fundamentally, hydrofoils differ from aerofoils in that two fluid phases are possible across a hydrofoil. The two phases include a liquid phase and a gas phase. The liquid phase is water and the gas phase is water vapor or air, either separately or in combination. When the gas phase present is predominately water vapor, the hydrofoil is cavitating. When the gas phase present is predominately air, the hydrofoil is said to be ventilating. If no gas phase is present, the hydrofoil is referred to as subcavitating.

Cavitation and ventilation both appear as bubbles attached to the surface of the operating hydrofoil. This phenomenon particularly occurs over the section back (suction side) of the hydrofoil with the bubbles varying both as to size and extent. The formation of vapor bubbles will occur within a liquid in a region where the static pressure of the liquid's flow field is equal to, or less than, the saturation (vapor) pressure of the liquid. The resulting low pressure is a consequence of the local acceleration of the liquid to a relatively high velocity over the hydrofoil surface.

In order for cavitation to develop, the surface pressures on the suction side of the hydrofoil must be lower than water vapor pressure. Ventilation will develop when surface pressures exist which are lower than the ambient pressure of an externally available gas supply. The gas supplied is usually air from an atmospheric source, although other sources may be employed.

Cavitation and ventilation are both ordinarily undesirable. While both cavitation and ventilation increase the section drag of the hydrofoil, cavitation is also barometrically unstable and can lead to problems such as vibration, excessive noise and erosion of the hydrofoil surface.

Whenever possible in the designing of hydrofoils, an attempt is made to avoid the occurrence of both cavitation and ventilation. In designing a hydrofoil for high speed applications, the development of cavitation and/or ventilation becomes increasingly difficult to avoid and if complete subcavitating conditions are insisted upon, the result is the sacrifice of low drag for adequate strength. For example, the achievement of complete subcavitating conditions in the hydrofoil sections of a planing boat propeller, having both reasonable efficiency and adequate strength, is generally impossible.

Aside from the problem of cavitation, conventional hydrofoil craft have the problem that their foils invariably operate in a turbulent flow regime, so that the drag on the foils is high, and a great deal of power is required to drive a conventional hydrofoil craft at high speeds. The hydrofoil craft has not always been popularly employed in a large number on sea although such craft have been known for many years. This is mainly because horse power needed to arrive at a gliding or planing state is very large and accordingly, such craft must be equipped with a motor much larger than a general ship. In addition, there were difficulties in powering hydrofoil craft, because quite a long propeller shaft was required to keep the propeller under water when the vessel rode onto the foils. The long shaft with its attendant bearings and mounting brackets added frictional drag and contributed to the expense of constructing hydrofoil craft, with the result that they were slower, less economical of fuel, and more expensive than they otherwise would be.